Franklin Genin

Dynamics of sonic jet injection into supersonic crossflow

The interaction between a sonic air jet and a supersonic air crossflow is simulated using large-eddy simulation (LES). A hybrid numerical methodology is used here to capture shock waves locally with minimal dissipation of the turbulent structures. The dynamic subgrid closure model employed for the LES permits a fully localized evaluation of the closure coefficients, such that there are no ad hoc adjustable parameters. Simulation of the experimental study of Santiago and Dutton (J. Propul. Power, vol. 13, 1997, pp. 264–273), where detailed measurements of the mean velocity and turbulent fluctuations have been acquired, is reported. The LES results show fairly good agreement with the experimental data for the mean and statistical fluctuations of the velocity field. The numerical study is then extended to two other jets in crossflow conditions to study the impact of the free-stream Mach number and of the jet to free-stream momentum ratio on the structure of the jet and on the dynamics of the interaction. The...

Subgrid Mixing Modeling for Large Eddy Simulation of Supersonic Combustion

Large eddy simulations of compressible mixing layers have been conducted to investigate scalar mixing at two convective Mach numbers at 0.25 and 0.62. Two sub-grid scalar transport models, one based on a conventional gradient di usion and the other based on the Linear-Eddy Mixing (LEM) model are used to predict scalar mixing in supersonic ows. It is found that the mixing layer growth is reduced signi cantly with increase in compressibility, which is consistent with past observations. Numerical predictions obtained using LES-LEM compares very well with the experimental measurements of scalar properties, whereas, the gradient di usion closure shows signi cant di erences from the measured values. Flow visualizations of density, temperature and mass fraction contours reveal the delay in the formation of the large structures and the growth of the mixing layer as the convective Mach number is increased. Statistics such as mean and the RMS of the velocity and the scalar eld exhibit self similarity in the far eld. PDFs of the species mass fraction in the supersonic stream become narrow as the compressibility increases, indicating the reduction in mixing.

Simulation of Cellular Detonation Structures in Ethylene-Oxygen Mixtures

Regular and irregular cellular detonation structures for stoichiometric ethylene-oxygen mixtures are simulated and studied. The structure of both two-dimensional (2D) and threedimensional (3D) detonations are considered and simulated using a simplified chemical model with Arrhenius kinetics. The motion of the triple point lines on the detonation front and the occurrence of the “slapping waves”, in case of 3D simulations, is presented. The effect of turbulence-detonation interaction on the detonation front and the presence of finer scales and the distortion of the structures produced by detonation due to turbulence are discussed. Further more, the mass fraction of the reactant downstream of the detonation front is analyzed. The enhancement of mixing and the formation of unreacted gas pockets downstream of the front due to turbulence is addressed in particular.

Large Eddy Simulation of Re-shocked Richtmyer-Meshkov Instability

Richtmyer-Meshkov instability is caused by the impulsive acceleration of two different media, and the instability causes the formation of small scales of turbulence structures. When the initial shock is reflected and passes again through the perturbed field, additional scales of turbulence larger than perturbation are generated. Here, direct numerical simulation of 2D RMI is used first to investigate flow structures similar to previous experimental and numerical studies and, comparisons show good agreement. Also, spectra of kinetic energy followed k 3 , which is commonly accepted to the 2D turbulence. Large-eddy simulation of 3D Richtmyer-Meshkov instability is also conducted using a localized subgrid closure. Even though mixing growth rate before reshock is overestimated, the growth rate after reshock shows good agreement with the experimental results. Vorticity structures are visualized by iso-surface of Q-criteria, and two regimes with strong vorticity are found at the edges of mixing region. The kinetic energy spectra shows an inertial range of k 5/3 at the later stage of reshocked turbulent decay. However, spectra of density showed no or only a small range of inertial range, which is shallower than k 5/3 .

Simulation of Detonation Propagation in Turbulent Gas-Solid Reactive Mixtures

The present study focuses on the simulation of highly reactive two-phase flows associated with detonations. A hybrid scheme, that combines a high-order shock-capturing method and a second/fourth-order accurate, low dissipation scheme is first validated by simulating blast wave discharges and shock-vortex interaction. The Large-Eddy Simulation model used in the present formulation is considered afterward in a fundamental study of shock-turbulence interaction. The solid phase tracking algorithm is then validated in supersonic environment. Finally, a detonative mixture composed of H2/O2/Ar, similar to an experimental configuration, is simulated with and without the presence of solid aluminum particles. The explicit role of the particles in this environment is considered, and the presence of a Double− Fronted Detonation is analyzed.

Characterization of Liquid Fuel Mixing in a Scramjet Flowfield

This paper describes an experimental program to provide high quality, fuel-air mixing data in a wellcharacterized scramjet model flowfield for validation of computational tools, such as subgrid LES models. The study employs a noncombusting, supersonic windtunnel and a standard, backward-facing step configuration that allows injection of gaseous or liquid fuel (or fuel surrogates). The facility is modular, with nozzles for providing flows from M=1.5 to 3.5. The facility is instrumented so as to provide a rigorous set of inlet, boundary, and internal flow properties. Results are presented for a Mach 2.5 flow without injection, and with 90° injection of a nonevaporating liquid fuel surrogate (acetone) just upstream of the step. The results include vertical profiles of inlet Mach number and static pressure; test section, wall static pressures; downstream profiles of flow static pressures; characterization of the droplet distributions produced by the fuel atomizer; and initial characterization of the fuel mixing as measured by laser-induced fluorescence images. The results are used to characterize the main flow features, such as the expansion over the step, boundary and shear layers and the reattachment shock. The results are repeatable, demonstrating the reliability of the facility. In addition, the liquid injection has little effect on most of the flow features, with results for this case nearly identical to the noninjection case.